a) $_REQUEST
  i)  $_POST(get data through <form>)
  ii) $_GET(get data through URL)

b) $_FILES (get details of uploaded files through browse button)

c) $_COOKIE (get cookie details)

d) $_SESSION (get session details)

e) $_SERVER (get server details)

f) $_ENV (get environment details)

PS: All the superglobal variables are an associative array.

We can print those variables as print_r($_SERVER);
<?php
echo "<br>=================<br>";
echo "<pre>";
print_r($_SERVER);
